As a welding joint of this kind, known is a welding joint disclosed in Patent Literature 1. The literature relates to an apparatus for welding a welding joint in which, when a synthetic resin-made welding joint and a synthetic resin-made tube are to be welded together, the welding joint can be fixed so as not to be displaced. According to the disclosure, a joining portion in which a pipe end portion of the welding joint and an end portion of the tube are fitted together is externally surrounded and heated by an annular heater (welding head) having a half-split structure, whereby the both end portions are welded together to integrally couple the joint with the tube.
In the above-described conventional art, the joining portion is externally surrounded so that it is just externally fitted by the annular heater, and it is advantageous that the joining portion can be efficiently heated. However, it has been known that the art has a chronic problem. Namely, the problem is that, when means for heating resins to melt and integrally weld together is used, the welded portion expands, and hence the joining portion wholly expands toward the radially inner side, or a weld bead which projects toward the radially inner side is formed, and a fluid passage is narrowed to impair the passage.
As a countermeasure against the problem, as disclosed in Patent Literature 2, a technique has been developed in which heating and welding are enabled to be performed without forming a bead or an expanded portion inside a joining portion, by ingenuity that, in both joining portions of a welding joint and a tube, each of end surfaces is formed by a plurality of sectional surfaces so that a gap is formed between them in a butted state.
However, the formation in which the pipe end portions of the welding joint and the tube are formed into a complicated shape involves large costs, but not always functions as expected. The case where a bead or an expanded portion is formed still sometimes occurs. In certain instances, a recess may be formed. There remains room for further improvement in smoothing an internal flow path in the joining portions.
As a welding apparatus of this kind, known is an apparatus disclosed in Patent Literature 3. In the welding apparatus, a pair of clampers (30) are supported so as to be swingable between a closing position where a joining portion (T1) is clamped from the both sides through a pair of heat conductive members (20), and an opening position where a gap between the pair of heat conductive members (20) is widened, a pair of heaters (40) are formed so as to extend along the pair of heat conductive members (20) by planer resistance heating members, respectively, and the joining portion (T1) is heated through the pair of heat conductive members (20).
Namely, the heaters (40) have a substantially Ω-like shape including a semicircular middle portion (41) which is placed on a radially outer side of the heat conductive member (20), and planar end portions (45) which are continuous to the both ends of the member. A lead wire is connected to one of the end portions (45), and the other ends are conductively connected in series to each other by a junk wire (67). The pair of heat conductive members (20) have a semi-tubular shape because of the cost and process easiness. Although description and illustration are omitted in Patent Literature 3, in order to prevent the end portions (45) of the heaters (40) from being short-circuited at the closing position, it is necessary that a physical gap of a certain degree is formed, and an insulation material is inserted into the gap. This tendency is similarly observed also in a welding apparatus disclosed in Patent Literature 4.
As a schematic diagram of a heating portion 140 at the closing position shown in FIG. 10, namely, circumferential gaps 147 are formed in the radially outer side of split surfaces (coupling surfaces) 146 of the heat conductive members 143 in the space between the end portions 142, 142 of the heaters 141 in the circumferential direction, and the insulation material (for example, a ceramic, mica, or glass) 144 is interposed. In the conventional example, because of the existence of the gaps 147 at the both ends of the half-split heaters 141, the joining portion 148 tends to be non-uniformly heated, so that the welded state is hardly stabilized, and the insulation material 144 must be periodically maintained and inspected, so that the heaters are not maintenance-free and their handling is cumbersome.
In welding joints of this kind, many joints having a shape other than a linear shape, such as an elbow shape (L-like shape), a T-like shape, and a Y-like shape are used. Among welding joints disclosed in Patent Literature 5, for example, an elbow shape is shown in FIG. 25, and a T-like shape is shown in FIG. 26. In such a welding joint having a shape other than a linear shape, i.e., a non-linear shape, in order to provide sufficient durability also in various usage modes, it is requested to withstand a severer acceleration test machine in tests in the productization stage.
Usually, a synthetic resin-made welding joint is produced by mold forming. In a welding joint having a T-like shape, a port for supplying molten resin, i.e., a so-called sprue gate is disposed in a body portion functioning as a base for three pipe end portions, and, in a welding joint having an elbow shape, in a curbed body portion through which a pair of pipe end portions are connected to each other. It has been known that, when such a welding joint is applied to the acceleration test machine, a crack or a break is easily formed along the sprue gate portion or a parting line in an early stage.
It is contemplated that, in order to improve the above-described early damage, means for thickening a sprue gate and the vicinity thereof in place of making uniform the thickness of a welding joint is effective. In this case, a thickness gradually changing structure in which the thickness is gradually increased is preferably used from the viewpoint of avoiding stress concentration as far as possible. However, there is an advantage which can be obtained by daringly forming a sudden change in thickness. Namely, as shown in a comparative example of FIG. 14, for example, the advantage is that, in a welding joint 260 of the T-like shape, a step 203a formed by the thickness difference between a joint body portion 203 and a pipe end portion 241 can be used as positioning means in a welding apparatus 270. This is a rational structure which can exert also an effect that means (the increased thickness of the joint body portion 203) for preventing an early damage from occurring can be used also as means for positioning in welding.
Namely, it is rational that means for holding the pipe end portion by a second sidewall 280 having an inner circumferential surface 280a of a width which is large to some extent in the direction of the axis 225p, to clamp the pipe end portion, as shown in FIG. 14, is used as means for grasping the pipe end portion 241 in the welding apparatus 270, and the second sidewall 280 is caused to abut against the step 203a for the purpose of positioning. In addition, a space portion 281 functioning as a minimum buffer region for relaxing the heat effect is formed in a gap between heating means (heater) 214 for welding and the second sidewall 280 in the direction of the axis 225p. Therefore, the length of the pipe end portion 241 in the direction of the axis 225p excluding the portion corresponding to the heating means 214 must be equal to or larger than a projection length 295 (=246+297) which is a sum of the width 297 of the second sidewall 280 and the width 246 of the space portion 281.
Although, as described above, the improved welding joint (the welding joint 260 shown in FIG. 14) having the rational structure which can attain at once both (a) the effect of preventing an early damage from occurring in the sprue gate portion or the parting line, and (b) the effect of functioning also as welding positioning means is assumed, it seems that the pipe end portion 241 is unnecessarily prolonged. Therefore, it seems that there remains room for further ingenuity, such as that the pipe end portion 241 can be compactified by making further improvement.
As an apparatus for welding a resin pipe and welding method of this kind, known are an apparatus and method disclosed in Patent Literature 6. Namely, the apparatus has a welding head (10) including a clamper (30) which can position and hold a resin joint (100), and a heating portion (40) which heats a joining portion (J). The clamper (30) has a joint holding portion (31) which is fitted to a position projecting portion (110) of the resin joint (100), and is supported so as to be swingable between a closing position where the clamper is closed so that a joint holding portion (31) is fitted to the position projecting portion (110), and an opening position where the clamper is opened so that the fitting can be cancelled. Then, the heating portion (40) is placed so as to, at the closing position, surround the joining portion (J). The joining portion (J) is heated and welded by the heating portion (40) while holding the resin joint (100) is positioned by the joint holding portion (31) in a state where the resin joint (100) is positioned by the joint holding portion (31).
The above welding apparatus and method will be described in a simplified manner. As shown in FIG. 21, a welding apparatus 370 has a joint holding portion 375 which externally surrounds and holds a welding joint 374, and a tube holding portion 376 which externally surrounds and holds a tube 381, and heating means (heater) 377 in a state where it is fitted onto a holder 384 which is externally attached to a pipe end portion 374A of the welding joint 374 is placed between the holding portions 375, 376. The joint holding portion 375, the tube holding portion 376, and the heating means 377 are configured in a known structure where lower structural members 375k, 376k, 377k and upper structural members 375j, 376j, 377j are pivotally coupled together, and the three members 375, 376, 377 are integrally swung to be opened and closed.
When a joining portion 382 of the welding joint 374 and the tube 381 is to be melted and welded together, an inserting step of inserting (internally fitting) an end portion 381A of the tube 381 into the pipe end portion 374A of the welding joint 374 onto which the holder 384 is fitted is first performed. Then, a setting step of positioning and placing the welding joint 374 accompanied by the tube 381 on the lower structural members 375k, 376k, 377k in a state where an annular ridge (positioning annular rib) 374B of the joint is fitted into a semiannular recess groove 388 of the lower structural members 375k of the joint holding portion 375 is performed.
Then, a checking and inserting step of inserting the tube 381 into the welding joint 301 until an end surface 381t which is the tip end surface of tube abuts against a step side surface 374c of the pipe end portion 374A is performed, and thereafter an attaching step of downward swinging the upper structural members 375j, 376j, 377j to be engagedly coupled to the lower structural members 375k, 376k, 377k, and causing the joint holding portion 375 to clampingly hold the welding joint 374, and the tube holding portion 376 to clampingly hold the tube 381 is performed. Then, a welding step of causing the heating means 377 to operate to melt the joining portion 382, and integrally welding the pipe end portion 374A of the welding joint 374 with the end portion 381A of the tube 381 is performed.
By the way, in order to correctly perform the welding, it is essential to, in the checking and inserting step, set the tube 381 to a state where the tube end surface 381t abuts against the step side surface 374c of the welding joint 374 and the both surfaces 381t, 374c are in contact with each other (the state which is drawn in the side of the sheet that is lower than the center line 385 in FIG. 21), and perform the attaching step under the abutting state. However, because of defectiveness of the checking and inserting step in which checking and inserting are frequently performed in a fuzzy manner, or a fault in which, in the attaching step, the tube 381 is slightly displaced in the pulling out direction, as drawn in the side of the sheet that is higher than the center line 385 in FIG. 21, the step side surface 374c and the end surface 381t are sometimes welded while they are separated from each other in the direction of the center line 385 and a gap (annular gap) 389 is formed.
When the gap 389 is formed, a disadvantage such as that a fluid stagnates therein, or that a foreign material or dust collects therein occurs. In the case of a fluid in which high cleanness is requested, such as cleaning liquid for semiconductor devices, or chemical liquid, particularly, a serious problem in that the purity is lowered by stagnation may be caused. In the conventional apparatus and method in which a tube is welded to the joint body at the initial position depending on the feeling of the worker at the site as described above, the welding is uncertain, and there remains room for improvement.